There are a variety of definitions of educational technology.
The Association for Educational Communications and Technology (AECT):
Educational technology is the study and ethical practice of facilitating learning and improving performance by creating, using and managing appropriate technological processes and resources. 
The Encyclopedia of Educational Technology:
Educational technology is a systematic, iterative process for designing instruction or training used to improve performance. 
Educational Technology involves the disciplined application of knowledge for the purpose of improving learning, instruction and/or performance. 
History of the Definitions
Edtech’s [educational technology’s] definition has evolved over the years as a variation of ways of dealing with learning processes (2), a conceptual framework (9), theory and practice (5), and the latest study and ethical practices of dealing with technological processes and resources (1). 
Audiovisual communications is the branch of educational theory and practice concerned with the design and use of messages which control the learning process. It undertakes: (a) the study of the unique and relative strengths and weaknesses of both pictorial and nonrepresentational messages which may be employed in the learning process for any reason; and (b) the structuring and systematizing of messages by men and instruments in an educational environment. These undertakings include planning, production, selection, management, and utilization of both components and entire instructional systems. Its practical goal is the efficient utilization of every method and medium of communication which can contribute to the development of the learners’ full potential. 
Educational technology is a field involved in the facilitation of human learning through systematic identification, development, organization and utilization of a full-range of learning resources and through the management of these processes. 
Educational technology is a complex, integrated process involving people, procedures, ideas, devices and organization for analyzing problems and devising, implementing, evaluating and managing solutions to those problems involved in all aspects of human learning. 
Instructional technology is the theory and practice of design, development, utilization, management and evaluation of processes and resources for learning.
- Richey, R. C., Silber, K. H., & Ely, D. P. (2008). Reflections on the 2008 AECT Definitions of the Field. TechTrends, 52(1), 24-25.
- Ely, D.P. (1963). The changing role of the audiovisual process in education: A definition and a glossary of related terms. TCP Monograph No. 1. AV Communication Review, 11(1). Supplement No:6.
- Association for Educational Communications and Technology. (1972). The field of educational technology: a statement of definition. Audio-visual Instruction, 17(8), 36-43.
- Association for Educational Communications and Technology (1977). The definition of educational technology. Washington, D.C.: Association for Educational Communications and Technology.
- Seels, B. B., & Richey, R. C. (1994). Instructional technology: The definition and domains of the field. Washington, DC: Association for Educational Communications and Technology.
- The Encyclopedia of Educational Technology. What is Educational Technology? Retrieved from: http://www.etc.edu.cn/eet/eet/articles/edtech/index.htm
- Spector, J. M. (2015). Foundations of educational technology: Integrative approaches and interdisciplinary perspectives. Routledge.
- Hsu, Y. C., Hung, J. L., & Ching, Y. H. (2013). Trends of educational technology research: More than a decade of international research in six SSCI-indexed refereed journals. Educational Technology Research and Development, 61(4), 685-705.
- Davies, I. K., & Schwen, T. M. (1972). Toward a Definition of Instructional Development.